Acoustically transparent sandable coating

ABSTRACT

A formulation for an acoustically transparent coating for use on an acoustical substrate comprising a water dispersible binder and relatively large filler particles, and characterized by a high pigment to volume concentration, high viscosity, high shear thinning, and fast drying to enable the formulation to be sprayed in droplets that retain their identity when in mutual contact and in a dry coating can be efficiently sanded flat.

BACKGROUND OF THE INVENTION

The invention relates to acoustically transparent coatings useful forimproving the appearance of acoustical substrates.

PRIOR ART

U.S. Pat. Nos. 8,684,134, 8,770,345 and 8,925,677 disclose drywallsheets or panels used to construct monolithic acoustical ceilings. Thepanels are perforated and clad with a thin, porous, translucent web suchas a non-woven translucent fiber glass veil or scrim. Joints between thepanels can be finished with tape and joint compound in the same manneras ordinary drywall is taped per gypsum association publicationGA-216-2013.

There is a need for a coating that can be applied across the drywallpanels to conceal the taped joints and the perforations visible throughthe veil and that does not significantly diminish the sound absorptionafforded by the panels. It is important that the coating can be appliedwithout requiring excessive skill on the part of a painter or tradesmanand that a full coating will dry within a limited time, preferably lessthan half of a single work day.

SUMMARY OF THE INVENTION

The invention provides a finish or appearance coating for an acousticalsubstrate that is characterized by a high porosity with a resulting highacoustical transparency and that can be efficiently sanded for a smooth,uniform appearance. The coating is particularly useful for concealingthe perforations in drywall sheets clad with a translucent orsemi-transparent non-woven veil and taped joints between such sheets ina monolithic acoustical ceiling construction.

The disclosed coating material is adapted to be sprayed in droplet formin multiple layers that are allowed to dry before a successive layer isapplied. The resulting coating structure is a three dimensional matrixof residual droplets and intervening voids or pores. The coatingmaterial is characterized by relatively large filler particles, notablyhigh pigment volume concentration, substantial shear thinning, and fastdrying properties.

The coating material is sprayed in a form of relatively large droplets.Owing to the physical properties of the material, the droplets retain atleast some of their individual character once deposited on the substrateor the previous layer of the material. The droplets, due to theirviscosity and quick drying properties, do not fully merge with adjacentdroplets. The droplets retain sufficient individuality to provide openspaces between the droplets of the same coating layer as well as theprevious coating layer. The open spaces, which are small enough to beoverlooked by the unaided eyes of a casual observer in a ceilingapplication, interconnect through the separate coat layers providing aporosity through the entire thickness of the coating.

The disclosed coating formula, besides affording acousticaltransparency, can be readily sanded to provide a relatively smoothappearance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary, schematic, isometric view of a gypsum panelacoustical monolithic ceiling employing the invention;

FIG. 2 is a fragmentary, cross-sectional view on an enlarged scale ofthe monolithic acoustical ceiling of FIG. 1;

FIG. 3 is a scanning electron microphotograph representing a face of theinventive coating;

FIG. 4 is a generalized graph of the viscosity of a formula of theinventive coating;

FIG. 5 is a graph of typical air flow resistivity of the inventivecoating; and

FIG. 6 is a photograph showing the outer layer of the inventive coatingas first dried on the right and after sanding on the left.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a schematic partial view of anacoustical monolithic ceiling installation 10. Portions of layers of theceiling 10 are peeled away to reveal constructional details. The ceiling10 is a suspended system including a drywall grid 11, known in the art,comprising main tees 12 spaced on 4 ft. centers and intersecting crosstees 13 spaced on 16 in. or 2 ft. centers. Dimensions used herein aretypically nominal dimensions and are intended to include industryrecognized metric equivalents. The main tees 12, to which the cross tees13 are interlocked, are suspended by wires 14 attached to asuperstructure (not shown). A perimeter of the grid 11 is conventionallyformed by channel molding 15 secured to respective walls 16.

Acoustical panels 20 are attached to the lower sides of the grid tees12, 13 with self-drilling screws 21. The illustrated acoustical panelsare 4 ft. by 8 ft. in their planar dimensions, but can be longer,shorter and/or of different width as desired or practical. The size ofthe panel 20 and spacing of the grid tees 12 and 13, allows the edges ofthe panel to underlie and be directly attached to a grid tee, assuringthat these edges are well supported.

Referring to FIG. 2, the acoustical panel 20 is characterized with aperforated gypsum core 24. One method of providing the core 24 is tomodify a standard commercially available sheet of drywall by perforatingit through a front paper face 23, the gypsum core 24, and a rear paperside or face 25. Perforations 28 can be formed by drilling, punching, orwith other known hole-making techniques. The perforations 28 arepreferably uniformly spaced; by way of an example, the perforations canbe round holes of 8 mm diameter on 16 mm centers. This arrangementproduces a total area of the perforations substantially equal to 20% ofthe full planar area of a panel 20. Other hole sizes, shapes, patternsand densities can be used. For example, tests have shown that a holedensity of 9% of the total area can achieve good results. Marginalareas, as well as intermediate areas corresponding to centers of supportgrid, joists, or studs, of a sheet can be left unperforated to maintainstrength at fastening points.

Sheets 29, 30 are laminated to both full sides of the perforated drywallsheet thereby at least partially closing both ends of the perforations28. At a rear side of the drywall, the backer sheet or web 30 ispreferably an acoustically absorbent non-woven fabric known in theacoustical ceiling panel art. By way of example, the backer fabric canbe that marketed under the trademark SOUNDTEX® by FreudenbergVliesstoffe KG.

At a front side of the drywall sheet 22, a sheet or web in the form of anon-woven fabric scrim layer 29 is attached with a suitable adhesive.The facing layer or sheet 29 is porous; a suitable material for thisapplication is that used commercially as a cover or face forconventional acoustical ceiling panels. An example of this type of veilmaterial is that marketed by Owens Corning Veil Netherlands B.V. underthe product code A125 EX-CH02.

The panel 20 with other identical panels is hung on the grid 11 in thesame manner as ordinary drywall is installed. Similarly, as shown inFIG. 1, joints 33 are taped in the same way as regular drywall is taped.Drywall joint compound or similar material 34 is used to adhere a tapeor similar material 35 to adjacent margins of two abutting panels 20 byapplying it directly to the sheets 29 and over the tape 35 to concealthe tape.

After the joint compound 34 has been sanded or sponged smooth, the frontsheets 29 and remaining joint compound are painted with an acousticallytransparent coating 31 of the invention. When the term monolithic isused herein, it is to denote that essentially the entire visible surfaceof a ceiling or wall appears to be a seamless expanse without joints.

The coating 31, in accordance with the invention, is sprayed onto theroom facing side of the panels 20 clad with the scrim or veil 29 whichconstitute a substrate for the coating. The coating 31, ideally, hasseveral physical properties including relatively large filler particles,notably high pigment volume concentration (PVC), high viscosity, shearthinning, and fast drying performance. When used as an appearance coatfor ceiling structures, the coating 31 should dry white.

A formulation for a suitable coating includes a binder and relativelylarge particles serving as a pigment or filler of a size larger thanthat ordinarily used in conventional paint-like coatings. The binder canbe a resin or polymer binder in or useable as an aqueous dispersion suchas, but not limited to, acrylic, styrene, or vinyl polymer latex oroil-based media such as an alkyd, polyurethane, polyester, or epoxy andcombinations thereof.

The relatively large particles include, but are not limited to, glassspheres, perlite, ceramic, fly ash, polymeric spheres, borosilicate,coarse sand, silica, and coarse carbonate. The relatively largeparticles are spherical or three-dimensional in shape without aprominent plate-like character or without a primarily columnar shape.The size of the large particles can range between 20 and 900 microns.

Table 1 below sets out an example of a suitable formula for theinventive acoustically transparent sandable coating 31.

TABLE 1 WEIGHT % MATERIALS SOURCE WEIGHT % RANGE FUNCTION Water 37.49(10-70) Solvent Natrosol HHXR DOW Chemical 0.41 (0-2) Thickener Tamol1254 DOW Chemical 0.29 Dispersant Titanium Dioxide Tioxide 6.11 FillerHaltech HP 42-296 Haltech 14.68 (0.05-80)   Latex Binder CalciumCarbonate Omya 29.60  (0-60) Filler Mica P80F USG 3.70 Filler TroykydD01 Troy Chemical 0.19 Defoamer Texanol Eastman 0.37 Coalescent ChemicalCo. Mergal 174 II Troy Chemical 0.16   (0-0.3) Biocide 3M K1 3M 7.00 (1-25) Filler 100.00

The function or role of the various constituents is noted in the columnon the far right. Water, thickener, binder and large particle filler areessential and the biocide is essential while the formula resides in acan. The remaining constituents can be considered optional. Forreference purposes, the titanium dioxide has an average particle size ofabout 0.6 microns; the calcium carbonate can have an average particlesize of about 12 microns, and the plate-like mica can have majordimensions of about 20 microns.

The 3M K1 material is glass bubbles (hollow spheres) of soda-limeborosilicate with a reported true density of typically 0.125 grams/cc.and an average particle size of 65 microns. The 3M K1 material isparticularly useful in practice of the invention. This material occupiesmuch of the volume of the formulation owing to its relatively lowdensity resulting from its hollow sphere structure. The K1 material,additionally, has a relatively low oil absorption of 0.2-0.6 gramsoil/cc of product per ASTM D281-84. Still further, the K1 material has anatural white color which is useful in ceiling applications. Thetitanium dioxide serves as an opacity improving filler and whiteningpigment. The calcium carbonate increases the solids and density of thecoating to further improve drying time and hardness of the driedcoating.

The pigment volume concentration (PVC) of the coating formulation, basedon all the solids, is relatively high and ideally is between 75% and 95%while alternative formulations will fall in a wider range of about 50%to about 99%. Generally, there is insufficient binder when the coatingis dry to fill the voids between the particles in the coating allowingair gaps in the coating. The ratio of the volume of relatively largeparticle fillers to fillers of more conventional lesser size can rangebetween about 0.1 to about 10.

The coating formulation of Table 1 has a relatively high viscosity ofabout 190 BBU (Brabender Units) to about 265 BBU at room temperature.Alternative formulations can have a viscosity of between about 66 BBU toabout 1450 BBU at room temperature.

The coating formulation of Table 1 is, and alternative formulationsshould be, characterized by a high rate of shear thinning. The shearthinning characteristics of the Table 1 formulation is generalized inFIG. 4 where by the measured viscosity drops more than 40:1. Theviscosity, in part, is contributed by the thickener component of theformulation. The listed thickener is cellulosic, but known synthetic andnatural thickeners can be used. The thickener also serves to keep theconstituents in suspension. The generalized viscosity chart of FIG. 4was measured on a Brookfield HA viscometer using a T-bar:C spindle.Alternative coating formulations should have a high shear thinningproperty so that the viscosity can reduce by a factor of at least 10 to1 under shear conditions.

The coating of Table 1 or an equivalent is preferably applied to thetaped, veil clad, perforated drywall panels or other substrate in aspray process of separately sprayed layers. Each layer or coat isallowed to substantially dry before a subsequent coat is applied.Preferably, the wet coating material is sprayed in large droplets using,for example, a conventional hopper texture sprayer such as that marketedby Graco. Ideally, the gun and/or its air supply is adjusted so that thedroplets are about 1 to about 4 mm. in size and, preferably, about 2 mm.in size. The shear thinning property, characterized in FIG. 4, of thewet coating allows it to be readily sprayed in droplet form despite itshigh viscosity. A single coating layer should involve spraying thesubstrate in two perpendicular directions or passes so that a uniformcovering by the layer is achieved. Two to seven layers can be applied,with four layers being preferred, to conceal the perforations and tapedjoints of the ceiling. Coverage by the total coating layers can bebetween 15 to 35 square foot per gallon, with 25 square foot beingpreferred. The formulation of Table 1 or a like formulation has adensity of 4 to 11 lbs. per gallon with a preferred range of 5 to 6.5lbs. per gallon.

FIG. 3 demonstrates the porous nature of the inventive coating whenapplied to an acoustical substrate. Inspection of FIG. 3 shows that thedroplets retain an individual nature while the binder bonds the solidsto the substrate at the first layer and to adjacent droplets between andamong layers. The scale on the lower right-hand corner of FIG. 3indicates a measure of 1 mm. and droplets can be observed in the FIG.measuring more or less than 1 mm. Preferably, the dried droplets rangebetween ½ and 2½ mm. The original droplet size will shrink when thewater content evaporates.

As layers of the coating are deposited, a three-dimensional matrix ofdroplets is built-up. Voids, dark areas in FIG. 3, can be observedbetween the residual droplet forms. These voids are interconnectedthrough the thickness of the coating resulting in a porous andacoustically transparent coating. Voids or pores in FIG. 3 are generallyless than 0.5 mm in size so that at ceiling height they are not readilyobservable by a person standing on the associated floor. The lowabsorption rate of the major solids volume of the coating, in the Table1 formula afforded by the 3M K1 large particles, assures that thecoating will be fast-drying. The shear thinning property of the wetcoating allows it to be readily sprayed in droplet form.

The combination of high viscosity and fast drying properties enables thedroplets to beneficially retain their individuality and avoid completelymerging with adjacent droplets. The disclosed formulation of Table 1applied in the specified layers dries to the extent that it will nottransfer to a finger, i.e. is dry to the touch, most preferably after 20minutes for a first coat and 30 minutes on subsequent coats in anenvironment of 75 degrees F. and 50% relative humidity. Preferably,alternative formulations with fast drying properties will dry to thetouch in 60 minutes in a first coat. This fast drying property of thecoating is also important since it enables a painter to complete a jobon the same day.

FIG. 5 illustrates typical airflow resistivity as measured separatelyfor each successive dry coat of the Table 1 material. The resistivity isdetermined with an air flow of 2 liters per minute through a surfacearea of 3.14 square inches. The total dry thickness of the coatingtested in FIG. 5 was 0.031 inch. The initial resistivity reading at “0coats” is a measure of the resistivity of the veil 29. The dry coatingis preferably between 0.03 and 0.15 inches thick.

The right-hand side of FIG. 6 illustrates the sprayed and dried finishof the top (outer) layer of the inventive coating. Architects, interiordesigners, building owners and/or tenants may prefer a smoother finish.A finer texture shown at the left in FIG. 6 can be obtained with theinventive coating formulation, without loss of acoustical transparency,by a sanding process. The inventive coating, as a result of the notablyhigh pigment volume concentration formulation can be readily sandedwhile retaining acoustical transparency.

Before sanding, the coating surface is characterized by macroscopichills and valleys corresponding to residuals of spaced dropletsdeposited during the spray application process. When sanded flat, as inFIG. 6 at the left, the inventive coating is characterized with a majorpart of its surface area being essentially flat, ignoring very finesandpaper scratch marks, and comparatively small local valleys.

By way of example, a ceiling area of 400 square foot covered with theinventive coating can be manually sanded with a pole sander in 45minutes to obtain a flat surface represented at the left in FIG. 6. Thiscan be compared for reference to another formulation requiring as muchas 1 hour and 45 minutes to manually sand a 400 square foot ceilingarea. Dust particles not removed in the sanding process can be removedwith an air blower, jet, or vacuum, for example.

Sanding tests were conducted to objectively measure the sandability ofthe inventive coating requiring 45 minutes and the reference coatingrequiring 1 hour and 45 minutes of manual sanding of a 400 square footceiling area. The tests were conducted according to the ASTM D2486standard (Test Method A) with appropriate deviations/modifications.Sandpaper of 180 grit (CAMI Scale) was substituted for the specifiedbrush with a weight of 1 pound over the sandpaper, no “abrasive scrubmedium (liquid)” was used and no brass shim was used. The coating wasapplied to an acoustical substrate as described above. The inventivecoating requiring 45 minutes of manual sanding tested at 60-65 cycles toobtain a flat surface (represented on the left in FIG. 6) while thereference coating requiring 1 hour 45 minutes tested at 110-115 cyclesto obtain a flat surface. Based on these tests and experience theinventive coating formulation should sand flat, preferably in about 85cycles and more preferably in about 60-65 cycles under the describedmodified ASTM D2486 standard.

It should be evident that this disclosure is by way of example and thatvarious changes may be made by adding, modifying or eliminating detailswithout departing from the fair scope of the teaching contained in thisdisclosure. The invention is therefore not limited to particular detailsof this disclosure except to the extent that the following claims arenecessarily so limited.

What is claimed is:
 1. A method of forming an acoustically transparentcoating on an acoustical substrate comprising spraying multiple layersof droplets on the substrate, the layers being allowed to at leastpartially dry before application of a subsequent layer, the dropletshaving sufficient portions free of contact with adjacent droplets toprovide interconnected pores that extend through a full thickness of thecoating, allowing the coating to fully dry and thereafter sanding thecoating flat.